I am having a moment and I can not find an answer in my search.

I am covering a cake with Swiss Meringue Buttercream and then adding fondant/tylose flower details. If this is put in the fridge overnight with the flowers stay in shape or soften/melt?

I personally would wait to put the flowers on. I refrigerate with flat decorations all of the time, and have refrigerated with various 50/50 deco's but if your flowers are 3D like roses (except, maybe rolled roses...those would probably hold up) there is a distinct risk that they'll wilt. Unless they've been drying for some time and are very, very dry by now.